Shoko Iwasaki Applications Scientist Shimadzu Corporation Kyoto, Japan
Gilbert Vial Molecular Spectroscopy, Product Manager Shimadzu Scientific Instruments Columbia, MD
Consumers have shown heightened concern about contaminants being discovered in products such as foods and pharmaceuticals. Despite the publicity surrounding such incidents, and despite the rigorous controls in pharmaceutical and food manufacturing, it is difficult to eradicate this problem completely, as the causes of contamination may include multiple processes—such as contamination of raw materials at the time of purchase, contamination of the product due to deterioration of component parts on the production line, and contamination of the product by the consumer. The types of foreign matter are also diverse, including not only organic materials such as human hair, plastics, and rubber, but also oxides, metal fragments, and other inorganic substances.
Choosing techniques for a contaminant search depends upon where one looks, and at what time. For a visible or surface contaminant, microscopy works well.
Several microscopy options exist—each with their own strengths and weaknesses. For example, FTIR is a molecular spectroscopy technique that measures molecular vibrational energy—but is especially sensitive to polar bonds. However, Raman microscopy is more sensitive to non-polar bonds.
When used independently, each technique offers useful though incomplete information about a contaminant. In addition, the overall process is more complex because multiple instruments are required and setting up each instrument to measure the same sample location can be a time-consuming process.
However, you may not need to choose between one or the other. A combination of the two techniques now exists, allowing the user to acquire both an infrared spectrum and Raman spectrum at the same location without moving the sample. And switching from one technique to the other is as simple as clicking a button.
Infrared Spectroscopy and Raman Spectroscopy
In infrared spectroscopy, infrared light is irradiated on the sample and the amount of light absorbed at each wavelength (wavenumber) is measured. In contrast, in Raman spectroscopy, the light scattered by a sample when it is irradiated with light of a specified wavelength is measured, and the energy difference between the incident light and the scattered light (i.e., the Raman shift) is then calculated. Like the infrared spectrum, the Raman spectrum is based on the vibrational modes of molecules.
Both techniques are used for purposes such as identification of substances by comparison with known spectra and structural determination and quantitative analysis of molecules. However, the intensity and shape of the detected peaks differ in the two methods. Some molecular vibration modes appear in the spectra of each technique, but not the other. Therefore, the manifestation of these modes differs between IR spectroscopy, which is based on absorption, and Raman spectroscopy, which is based on scattering. While FTIR is a more common technique that is completely non-destructive, Raman spectroscopy can identify inorganics. When used together, they offer a more complete picture of a particular contaminant.
Experimental Setup
Figure 1 shows the appearance of a contaminant adhering to the surface of a tablet. The contaminant is reddish-brown and exists in a scattered form over a range of about 100 μm on the tablet surface. With conventional instruments, the single most time-consuming process is the work of setting the tablet on the microscope stage and adjusting the measurement position so that it is in the field of view (FOV) of the microscope. The AIRsight microscope, in contrast, incorporates a widefield observation camera, making it possible to observe a FOV at a size that is visible to the human eye (10 × 13 mm). The microscope camera, used in the actual measurement example, can observe areas as small as 30 x 40 μm, allowing observation of microscopic contaminants.
Positional information is shared by the microscope camera and the wide-field camera, ensuring that the FOV does not shift due to switching between the two cameras. This enables a more efficient analytical process as there is no need to spend time searching for the same position. Table 1 shows the measurement conditions.
Contaminant Analysis by Micro-Infrared Spectroscopy
Infrared spectra were acquired first to avoid damaging the sample. An analysis of the normal area and an area with an adhering contaminant was conducted by micro-ATR measurement. Figure 2 shows the acquired infrared spectra.
The infrared spectrum of the normal area corresponded to the main component (mannitol) of a pharmaceutical product. However, it was not possible to identify the cause of contamination, because no peaks were detected from the area with the adhering contaminant.
Contaminant Analysis by Micro-Raman Spectroscopy
Raman spectra were then acquired, and an analysis of the normal and contaminated areas was carried out by micro- Raman measurement. Figure 3 shows the measurement results of the Raman spectra. The differences between the spectra of the normal area and the contaminated area are clearly evident.
A Raman spectrum was acquired for iron oxide, which may possibly have adhered to the tablet surface, and the spectra of the contaminated part of the sample tablet and the iron oxide were overlaid, as shown in Figure 4. Since the two spectra showed close agreement, the contaminant adhering to the tablet surface was inferred to be iron oxide.
Infrared Spectrum of Iron Oxide
As seen in Figure 2, it was not possible to identify the cause of the contaminated area by micro-infrared spectroscopy. Figure 5 shows the infrared spectrum (measurement method: single reflection ATR) of iron oxide, which was estimated to be the contaminant by micro-Raman spectroscopy.
Because the characteristic peak in the infrared spectrum of iron oxide was located on the low wavenumber side from 510 cm-1, it could not be detected by micro-infrared spectroscopy using an T2SL detector. However, useful data could be obtained by Raman spectroscopy because this technique has higher qualitative capability for inorganic compounds than infrared spectroscopy.
Conclusion
Contaminant analysis of a pharmaceutical tablet was carried out by micro-infrared spectroscopy and micro-Raman spectroscopy using one instrument. Although trace amounts of inorganic compounds are difficult to analyze qualitatively by infrared spectroscopy, using one instrument that also performs Raman spectroscopy made it possible to identify an inorganic contaminant.
The ability to obtain infrared and Raman spectra of the same location using only one instrument, and one software platform that easily switches between the two measurements simplifies the investigative process and provides a more complete qualitative analysis of unknown samples. Using only one instrument also saves valuable bench space by being able to analyze mixtures of both organic and inorganic substances.
References
1. Hesamodin Hosseini Ghahi, Johannes Kiefer. Raman Spectroscopy: Multiple Techniques Can Analyze Pharmaceutical Tablets and Capsules, Tablets & Capsules, March 2022.
2. Gretchen L. Shearer, Ph.D. Contaminant Identification in Pharmaceutical Products, The Microscope, Vol 51:1, 3-10 (2003).